Spread band tuning circuit



May 17, 1949. 2. BENIN 2,470,426

SPREAD BAND TUNING CIRCUIT Original Filed 001;. 28, 1942 2 Shqe'ts-Sheet l May 17, 1949. z. BENIN 2,470,426

SPREAD BAND TUNING CIRCUIT Original Filed Oct. 28, 1942 2 Sheets-Sheet 2 Patented May 17, 1949 SPREAD BAND TUNING CIRCUIT Zolmon Benin, Glen Ellyn, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Original application October 28, 1942, Serial No. 463,665. Divided and this application August 22, 1945, Serial No. 612,039

3 Claims.

This invention relates to spread band tuning circuits and is particularly useful in radio receiving sets, but is applicable to any apparatus employing tuning circuits, such as signal gen erators, transmitters, etc.

This application is a division of the Patent No. 2,383,463 dated August 28, 1945, entitled Spread band tuning device, and assigned to the same assignee as the present application.

It is well known that various portions of the radio spectrum are much more crowded than other portions. For example, in short wave broadcast reception it is found that frequencies, and corresponding portions of the dial, from 9.5 to 9.7 megacycles and from 11.7 to 11.9 megacycles, being respectively the 31 and 25 meter short wave bands, are very crowded, whereas the frequency band from 9.7 to 11.7 megacycles contains very few stations. Likewise, the frequency bands from 15.1 to 15.35 and from 17.75 to 17.85 megacycles, being respectively the 19 and 17 meter short wave bands, are crowded, while the band from 15.35 to 17.75 megacycles contains practicall no stations. In conventional receivers wherein modified straight line frequenc tuning condensers are ordinarily employed to tune through. a relatively wide frequency band, it is found to be almost impossible to tune accurately a station in the crowded portion of the dial, for the reason that a very slight movement of the tuning control results in a considerable change in the capacity of the condenser, and accordingly, in the resonant frequency of the tuned circuit. Thus, a slight rotation of the dial is apt to tune the receiver through the frequencies of a number of stations, resulting in great difiiculty in accurately tuning a desired station and eliminating others.

The terms spectrum and .bands are used throughout this specification in their commonly accepted meanings, for example, the radio frequency spectrum includes all of the above mentioned allocated short wave bands, i. e., the 31, 25, 19 and 17 meter bands and the so called dead ranges of frequency similar to the range from 9.7 to 11.7 megacycles and 15.35 to 17.75 megacyoles mentioned above.

It is an object of this invention to provide a new and improved tuning circuit which produces rapid frequency changes in certain frequency ranges and slow frequency changes in other ranges, upon movement of the tuner at a uniform rate throughout its range of adjustment.

It is another object of the invention to provide such a tuning circuit which spreads the scale indications of those portions of the radio spectrum containing large numbers of stations and which condenses the scale indications of those portions which are little used.

A further object of the invention is to provide such a tuning circuit in which the inductive reactance is so modified locally as to produce desired rapid frequency changes in certain ranges and desired slow changes in other ranges.

In accordance with the invention the tuning circuit is arranged and constructed so that a single scale encompasses at least two separate crowded portions of the frequency spectrum, and so that these two separate crowded portions are expanded substantially uniformly, the intermediate portion of the frequency spectrum being condensed on the scale.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Figure 1 is a perspective view of a condenser constructed similarly to the present invention;

Figure 2 illustrates the shape of a single rotor plate of the condenser of Figure 1;

Figure 3 is a perspective view of two condensers mounted on a single shaft, one being constructed like that illustrated in Figure 1;

Figure 4 is a perspective view of a modified form of condenser similar to that illustrated in Figure 1;

Figure 5 illustrates a single stator plate of the condenser of Figure 4;

Figure 6 is a view, partly in section, of a variable inductance constructed in accordance with my invention;

Figure 7 is a modified form of inductance similar to that shown in Figure 6;

Figure 8 is a schematic diagram of a tuned amplifier circuit including a condenser, as illustrated in Figures 1 and 4;

Figure 9 is a schematic diagram of a tuned amplifier circuit including a variable inductance constructed in accordance with my invention, as illustrated in Figures 6 and 7;

Figure 10 illustrates a pair of tuning scales arranged to cooperate with a pointer adjustable simultaneously with the condenser of Figure 8 01' the inductance of Figure 9;

Figure 11 is a schematic diagram of a modified form of tuning circuit constructed so as to pro- 3 duce results similar to those produced by the circuits of Figures 8 and 9; and

Figures 12 and 13 show other modified forms of tuning circuits.

With reference to Figure 1, the condenser is provided with rotor plates Ill mounted on a shaft II, the rotor plates being adapted to be rotated into position between stator plates 12 which are secured together by suitable fastening means 13.

The effect of this type of rotor plate can best be described with reference to Figure 2. As the rotor plate is rotated from the position shown in Figure 1 into the space between the stator plates, it is apparent that segment A first enters this space. As the radius and the area of segment A are relatively large, a substantial change is produced in the capacity of the condenser for each increment of rotation. As the segment B begins to enter the space between the stator plates, the amount of change of capacity for a given rotary movement is reduced materially since segment Bis of small radius and area, and each increment of rotation introduces only a small additional area. As segment C enters the space between the-rotor plates, the capacity of the condenser again changes very rapidly for the same rate of rotation because of the large radius of the segment being introduced. Segment D produces a slow change of capacity at the same rate of rotation for the same reason as segment B. Segment E again produces a rapid change in capacity similar to that of segment A.

I'l1is invention is particularly adapted for use in short wave broadcast reception, since it is in the short wave broadcast band that local overcrowding of stations occurs. In a radio receiver designed to receive standard broadcasts, as well as short wave broadcasts, a second condenser, such as a modified straight line frequency condenser may be provided, which cooperates with more or less uniform markings on the standard broadcast dial. These two condensers may be separately provided. However, it has been found advantageous to combine the two condensers into a single unitary structure.

Such a combined unitary structure is illustratedin Figure 3, in which the modified rotor plates l9 and the standard rotor plates 20 are mounted upon-the same shaft H, and are all electrically connected. The stator plates I2 may be divided into two sections between which electrical insulation Z! is provided. That is, two separate con- 'densers may be built together, one serving for the standard broadcast range, and the other for the shortwave range. The rotor plates are usually connected to ground and, therefore, there is no necessity for electrically insulating the two sections of rotor plates of the condenser, although these sections may be insulated if rcciuired by the circuit employed. The electrically insulated stator sections of the condenser are connected to a suitable band changing switch (not shown) so that either one may be included in the tuned circuit, depending upon the station which it is desired to receive. As shown in Figure rotor plates exceed by one the number of stator plates, so that when the rotor plates are in engagement with the stator plates they surstator sections is minimized.

" In Figure 4 a-modification of this arrangement is illustrated in which the rotor plates ID are solid and in which the stator plates [2' have been perforated to produce the desired effect. The contour of a stator plate is clearly illustrated in Figure 5, wherein segments A1, B1, C1, D1 and E1 correspond to segments A, B, C, D and E in Figure 2. It is apparent that the working of this modification is the same as that of Figure 1, the effect being produced by the stator plates as distinguished from the rotor plates.

Figure 6 illustrates a variable inductance of the variable permeability type which may be used in accordance with my invention to produce an effect similar to that of the variable condensers of Figures 1 and 4. This variable inductance may include an insulating tube 3| about which a coil 32 is wound. The conductors forming the coil are uniformly distributed over the tube 3!. A core 33 of low loss magnetic material is arranged to move into and out of the tube 3|.

The configuration of the core is such as to produce the desired result, such as that produced by the condensers of Figures 1 and 4. The large section A2 at the end of the core 33 produces. a rapid change in inductance when inserted the tube 3| and accordingly ,a large change of frequency. As section B2 enters the tube 3!, the rate of change of frequency is lower, since the cross-sectional area of the core is smaller. Section C2, which is of large cross-section, produces a rapid change of frequency. Section D2, like section B2, produces a slow changeof fr quency because of its small cross section. Section E2 of large cross-section, like section C2, producesa rapid change of frequency. It is apparent that by suitably adjusting the cross-sectional areas of the sections referred to, this variable inductance of Figure 6 may be made to produce the same effect as the previously described variable condensers of Figures 1 and 4.

Figure '7 illustrates a variable inductance having a uniform core 36 arranged to move into and out of a non-uniformly wound inductance coil. The inductance illustrated is designed-to produce a result similar to the result produced by the reactances illustrated in Figures 1, 4- a'nd 6. It includes a tube 35 about which a conductor may be wound in sections, each section having the requisite number of turns to produce the desired rate of change of frequency for that section.

' Sections A3, C3 and E3 are closely wound and contain many turns per unit length. Sections Bi and D3 contain a few turns per unit length. It is evident that as a uniform core 36 enters the tube 35, there is produced in sequence a rapid change, a slow change, a rapid change, a slow change, and a rapid change of inductance. and therefore of frequency. v l l While the condenser as described above/and as illustrated in Figures 1 and 4, have been described as being constructed with either the stator plates or the rotor plates modified toes pand certain portions of the spectrum while com pressing other portions, it is apparentthat the same result can be produced by partially modifying both the stator plates and the rotor plates, so that the modified portions of both plates cooperate to produce the desired result. Similarly the inductances illustrated in Figures Tandy-8 achieve the desired result in the oneccase -by reason of a modified core shape and in the other case by reason of modified windingdistribution. It is apparent that the desired results according to the invention may be produced by making both core shape and winding distribution non-uniform so as to produce the same result described as produced by the non-uniformity of either one of these constructional features of the inductance.

In Figure 8, the condenser 9 is connected in shunt to an inductance 13', illustrated as a transformer, and is a condenser such as that illustrated in Figure 1 or 4. The condenser 9 and inductance 13' form a tuned input circuit for a stage of tuned amplification, in which the tuned frequency may be adjusted.

In Figure 9 there is shown a similar tuned amplifier in which the tuned input circuit includes an inductance 40, such as the inductances illustrated in Figures 6 and 7. This inductance 40 is connected in series with an inductance 41 which is illustrated as an input transformer, and a fixed condenser 42 is connected in shunt to the series combination of inductances 40 and 4|.

In Figure there is illustrated a pair of scales suitable for cooperation with a pointer attached to the movable element of the inductance 40 of Figure 9, or the condenser 9 of Figure 8. The upper of the two scales of Figure 10 shows that the portion of the frequency range from 9.5 to 9.7 megacycles has been spread out considerably to take care of the large number of stations in that area. Likewise, from 11.7 to 11.9 megacycles there are many stations, and this portion of the scale has also been spread out considerably. The portion from 9.7 to 1 .7, which contains few stations, has been crowded together. This upper scale of Figure 10 shows that the two standard short wave bands from 9.5 to 9.7 megacycles, and from 11.7 to 11.9 megacycles have been spread substantially uniformly and equally. Each of these two short wave bands covers 200 kilocycles, and each is spread for substantially the same linear distance along the scale. The segments B and D of the adjustable reactanoe in the tuning circuit are so formed as to produce this substantially uniform spread in each of the two separated spread band portions.

Comparing the shape of the condenser rotor plate illustrated in Figure 2 with the lower scale of Figure 10, it is evident that segment A produces a rapid change in frequency from 18.5 .to 17.85 megacycles. This insures complete coverage of the upper end of the range. When segment B is rotated into the space between the stator plates, the slow change of capacity and of frequency corresponds to the large distance on the scale between 17.85 and 17.75 megacycles. Then, as segment C enters between the stator plates, the rapid change in capacity produces a. large change in frequency corresponding to the relatively short distance along the lower scale from 17.75 to 15.35 megacycles. As segment D enters between the stator plates, its slow change of capacity corresponds to the long part of the scale between 15.35 and 5.1 megacycles. Segment E produces a rapid change of capacity from 15.1 to 14.5 megacycles so as to insure complete coverage of the lower end of the range.

In the previous paragraph the condenser has been described as covering the range from 14.5 to 18.5 megacycles. This condenser is also used to cover the range from 9.0 to 12.5 megacycles, by changing the inductance with which it is connected in shunt so as to make the tuned circuit resonant within the range 9.0 to 12.5 megacycles. That is, two inductances are used, one being connected in shunt with the condenser for the high frequency range and the other for the low frequency range. The circuit is switched 6 from one inductance to the other depending upon which wave band it is desired to tune.

Complete coverage of the range from 9 to 18.5 megacycles can also be accomplished by a single condenser without a change of inductance by forming a condenser plate so as to give rapid coverage of the ranges 9.0 to 9.5, 9.7 to 11.7, 11.9 to 15.1, 15.35 to 17.75, and 17.85 to 18.5 megacycles; and to give slow coverage of the ranges between 9.5 and 9.7, 11.7 and 11.9, 15.1 and 15.35, and 17.75 and 17.85 megacycles. That is to say, condensers or inductances as described hereinabove can be employed wherever it is desired to extend substantially uniformly selected portions of a tuning range while compressing intermediate portions of the range.

In the previously described circuits the desired results have been produced by forming one or more elements of the adjustable tuning reactanoe so as to give the desired non-uniform frequency change. In Figures 11, 12 and 13, similar results are produced by introducing into, or removing from the tuning circuit one or more reactances separate from the variable tuning reactanoe. This is brought about by switch means operated in unison with the tuning reactanoe and arranged to control the connection of additional inductance or capacity to the circuit such as to vary the rate of tuning in the various sections of the radio frequency spectrum in the desired manner while maintaining a continuous change of frequency across the entire range.

In Figure 11 there is illustrated a tuning circuit having a variable capacity 1| and a fixed inductance 12. Operatively connected to the variable condenser H are switches 13 and 14, The switch 13 is adapted to introduce additional capacity into the circuit while switch 14 is arranged to introduce additional inductance into the circuit. When the condenser 1| is adjusted to minimum capacity to tune the circuit to high frequencies in the desired range, switch 14 connects inductance 15 in series with inductance 12 across condenser 1|, the only capacity in circuit. Since condenser 1| is the only capacity in the tuning circuit, a change in its capacity produces a relatively rapid change of frequency.

At a desired point in the adjustment of condenser 11, as its capacity is increasing, switch 14 removes inductance 15 from the circuit and introduces another inductance 16, which is a smaller inductance. Simultaneously, switch 13 introduces condenser into the circuit in shunt to the condenser 1|. The condenser 80 and the inductance 16 are selected to be of such appropriate values that there is no change in the product of inductance and capacity, and therefore no change in frequency, as the switches 13 and 14 move from one contact to the next. Thus, the frequency immediately before the change is the same as the frequency immediately following the change. in shunt to capacity 1|, further adjustment of condenser 1| to increase its capacity results in a slower change of frequency because of the reduced rate of change of the overall capacity. This effect is used to spread out a desired portion of the spectrum, as from 17.85 to 17.75 megacycles. At the end of that portion of the spectrum, as condenser 1| is being adjusted in the direction of still greater capacity, switches 13 and 14 move to the next contact, thereby introducing inductance 11 into the circuit and removing condenser 80 therefrom. The inductance 11 is of suitable value to maintain the product of inaavonee ductance: and" capacity:- constant. andithereforeito keep the frequency the: same.-- as: it.was;im-'-- mediately before condenser -fifliwas-memovedrfrom theicircuit'... The inductance-.1 1. is11argenthan the inductance-16;: as-i is: necessary because the (3011+ denser 80:. has been; removed: from the circuit; With inductance. 11-v in the circuit, and condenser;

wroutzof thercircuit, thetchange-of frequency is. rapid; since achange in: the; capacity of: con--- de'nser 1=lrnow produces an. equal'ichange the: oyeralhcapacit'y of :thevtuning. circuit. The part:- of?thGEIfI'EQ'LIGHCYP'SP@Ctrunr covered by the tuning: circuit: withninducta-nce. 11-: in the circuit and. condenserz-aorout may, for example, be used to cover: a little: used portion of the spectrum 1 rap idly, as from 17.75 to 15.35 megacycles:

When condenser 11.- is increased incapacity until. the:-- tuning circuit is: tunedto: the lowest frequency in. this little. used range ofthe spectrum sw-itch; 1lledisconnects inductance] from the-.-circuit and introduces inductance 18, and si+- multa-neously switch. 13treconnects condenseriflih inwthe-circuit. The inductance 181s of thevalue;

necessary so that-these switching operations. producesno changein frequency of the tuning circuit.

Theinductanoe18 is smaller than the inductance 11. 130-: compensate for: the introduction of con denser 81] in the circuit.- Adjustment of the condenser. 1L with these connections-results. in: a

slow change -.offrequency for the .same reason as when-the inductance 16--andcondenser 30 were.

in circuit. The portion of thefrequency spectrum covered by adjustment-of the condenser 1[ under these; conditions may correspond, for example,

tmthatrportionibetween.15.35 and 15.1 megacycles.

Atathe end of that portion ofthe frequency spectrum,- switch 1A-=removes inductance 18sfrom.

circuitandintroduces: inductance 19 into the circuit, while: switch 13 removes condenser 80. from: theccircuit. This switching operation is alsolcarriediout without any change of frequency: because. the. inductance [9 is made: of proper.

value. Further increase of capacity of condenser. 11 produces a--.rapid change in frequency.

Itisevidentfromthe above descriptionthat: the circuit of-Figu-re. llproduces in sequence. a

rapid change,v a. slow change,v a rapid: change," a slow. change, and a. rapid change of frequency,

with uniform-change incapacity ofthe variable i It is. evidentthat the. shape of the 2 condenser 11-. platesofcondenser 1| -may be modified to produceqthezsubstantia-lly uniform frequency. change over the two...shr-.t.WaVe bands on each-.of the-:- soales...illustrated in .Figure. 10. If desired, sepa ratetfixed. condensers-for each of the five tuning. ranges correspondingato each of theinductan'cesv T5.-to. 19=may be. employed instead. of a singlecondenser 80 short circuits-inductance 89, so as to remove it from theecircuit. Inductance 89-and condenser 90-. areaof .suitable'. values so that the: frequency immediately. precedingthe switchingoperationis 8?. the: same; as thatl immediately: followings: the; switchingv operation.

After. condenser: :is'inserted and inductance 89 is removed; thera-te of change-of frequency;- withuchange: in capacityoftheva-riable condenser 85 is materially reduced: At the end of: the-vdesiredv spread-band portion, switch 81 removes: condenserzte'sfrom thecircuit andsubstitutes con-tdenser 9!, a smaller condenser. Simultaneouslwswitch 88 removes. the shorttcircuit from=the .inductance 89; sov that. it isagain eifectivezinzther circuit. Condenser 9-l*is of suitablevalueesothatw the switching; operation. is efiectedwithout. at change of frequency to Which-the circuit-istuned.

. Afterthis switching;- operation the raterotchange:

of frequency withchange of capacity oflthe ad justable condenser 85. is rapid.

As the capacity of condenser 85 is further in creased,v switch 8 1 removes condenser: 91 from the circuit andsubstitutescondenser 92, a larger c.on.- denser, in place thereof. Switch:88.again-.:removes inductance 89from the. circuit by. short. circuiting-itt Condenser 92 is-of suitable value; to maintain the frequency-thesame beforeand. after the switching. operation. Afterthisswitch ing operation .therate of change of frequency is. again. slow over a desired range; after. which. switch 81 removes .condenserfizfromth'e circuit. and introduces. condenser 93, a smaller. condenser. than 92. Simultaneously, .switchBBAremoves the. short circuit from inductance 89 so that it isagain. effective in the circuit. Condenser. 93Iis of suit? able value to maintain the frequency thesame. before and after the switching operation. There? upon further increase of capacity of the.cone denser 85 produces a rapidlchangeoflfrequenc e It is thus evident thatthis circuit also produces. in sequencea. rapid change, a slow change, a.- rapid "change, a slow change; and a rapid change of frequency.

In Figure 13 there isillust'rateda circuit arranged to produce the same efiect'and including; a variable inductance. such as those illustrated in. Figures Sand '1. This circuit includes .-a fiiced.

. condenser 9'5and a variable'induct'anceflfi. Operatively. connected to the inductance. 9'3 for move ment in unison therewith areswitches' 91 and98 At the h'igh'frequency end'of the portion of'the spectrum covered'by the tuningcircuit, induct ance 95Tfand condensers 95"and lUllareinsertedII With th'e'sezconnections a small change in inductan'ce produces a.large change in frequency; At a desired'frequency, switch 98 removes condenser. lmi'from thecircuit and at the same time switch eifi'places the additional'inductance' 99"in circuit in'series with inductance 9G; Inductance 99and condenser I 00 are of suitable values :so. that" the frequency with which" the tuning circuit is "res= onant'is the'samebefore and "after-the switching operation.

With the added inductance 99 in circuit; change of frequency upon change in the inductance 9 6 is-slow so as to spread'th'edrequencies of a de-' sired portion of=-the spectrum; At the end of 'that portiomofthe bandover which frequencies are so spread', switch 91 again removesinductance 99from the=circuit=by short circuitingand switch S8 'places a-rclatively largecondenser I 0 l in cil cuit'; the; condenser. I ilit.having-..a valueisuch' that a frequenoy changeisnotproduced bythe-switch ingaoperation.-

Further adjustment .of-.:inductancea 96Zproduces a: rapid change frequency-:w-ithrsuoh switching; connections.-. As the inductance further increases: switch 98 removescondensenl 01 from circuit and i substitutes therefor a smaller condenser l 92, while switch 9'! simultaneously replaces inductance 99 in circuit. Condenser I92 is of suitable value to maintain the frequency the same before and after the switching operation. This change in the circuit spreads another portion of the band, so that further increase of the inductance 9'5 produces a relatively slow change of frequency.

Upon still further increase in inductance 96, switch 97 removes the inductance 99 from circuit by short circuiting it, and switch 98 removes condenser I02 from circuit and substitutes a larger condenser Hi3 in its place. Again, there is no change of frequency before and after the switching operation. After this circuit change, change of inductance 96 produces a rapid change of frequency. It is evident that this circuit produces much the same effect as the circuits of Figures 11 and 12.

The circuits of Figures 11 through 13 are particularly suited for producing the results illustrated on the two scales of Figure 10. As pointed out previously, the spread-out portions of each of these scales are spread substantially uniformly and equally, the intermediate portions on each scale being compressed. It is evident that these circuits may be employed without band switching to produce over one scale all of the changes in tuning rate illustrated in Figure 10 on two scales by merely providing the requisite number of elements to cover the entire range.

In the drawings only those elements of tuning circuits necessary to produce an operative circuit have been disclosed. It is evident that additional refinements in tuning circuits may be employed in conjunction with the present invention without departing from the spirit thereof. The switches employed in the various circuits of Figures 11, 12 and 13 may be mounted upon the same shaft or other movable member used to tune the variable L tuning member, and may be of the snap-action type. However, these switches may be separate from the tuning member and may be operated therefrom by any suitable means such as gears, belt chain drives, etc.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. A tuning arrangement for a radio receiving system, and the like, for continuously tuning said system through a frequency spectrum and for simultaneously providing band spreading in selected bands of said spectrum, said arrangement comprising a tuning circuit including a first reactor, a variable reactor of opposite sign coupled in shunt with said first reactor for tuning said circuit through said spectrum, a first switching unit mechanically coupled to said variable reactor for selectively switching a plurality of further reactors of the same sign as said first reactor in circuit therewith, a second switching unit similarly coupled to said variable reactor for switching a further reactor of the same sign as said 10 variable reactor in circuit therewith at said selected bands to provide band spreadin therein, the values of the selected ones of said first mentioned further reactors being so related to the value of said second mentioned further reactor that the inductance-capacitance value of said circuit is constant at the switching points of said second mentioned further reactor, whereby said variable reactor continuously tunes said circuit throughout said spectrum.

2. A tuning arrangement for a radio receiving system, and the like, for continuously tuning said system through a frequency spectrum and for simultaneously providing band spreading in selected bands of said spectrum, said arrangement comprising a tuning circuit including an inductance, a variable capacitor coupled in shunt with said inductance for tuning said circuit through said spectrum, a first switching unit mechanically coupled to said variable capacitor for selectively switching a plurality of further inductances in circuit with said first mentioned inductance, a second switching unit similarly coupled to said variable capacitor for switching a further capacitor in circuit with said variable capacitor at said selected bands to provide band spreadin therein, the values of the selected ones of said further inductances being so related to the value of said further capacitor that the inductance-capacitance value of said circuit is constant at the switching points of said further capacitor, whereby said variable capacitor continuously tunes said circuit throughout said spectrum.

3. A tuning arrangement for a radio receiving system, and the like, for continuously tuning said system through a frequency spectrum and for simultaneously providing band spreading in selected bands of said spectrum, said arrangement comprising a tuning circuit including a fixed capacitor, a variable inductance coupled in shunt with said fixed capacitor for tuning said circuit through said spectrum, a first switching unit mechanically coupled to said variable inductance for selectively switching a plurality of further capacitors in circuit with said first mentioned fixed capacitor, a second switching unit similarly coupled to said variable inductance for switching a further inductance in circuit with said variable inductance at said selected bands to provide band spreadin therein, the values of the selected ones of said further capacitors being so related to the value of said further inductance that the inductance-capacitance value of said circuit remains constant at the switching points of said further inductance, whereby said variable inductance tunes said circuit continuously throughout said spectrum.

ZOLMON BENIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,904,165 Mullner Apr. 18, 1933 2,137,318 Wheeler Nov. 22, 1938 2,297,389 Brandholt Sept. 29, 1942 2,434,299 Van Loon Jan. 13, 1948 

